Power over Ethernet (PoE) refers to a technique of transmitting electrical power over twisted-pair cabling, along with data, to remote devices in an Ethernet network. PoE as standardized in IEEE 802.3af provides 44 to 57 volts over at least two-pairs of a four-pair cable at a current of up to 350 mA for a guaranteed load power of approximately 15.4 watts. It is also possible to provide power and data over electrical lines, over power buses, and the like. As used herein, the term “powered network” refers to system that delivers power and data on a cable, comprised of one or more wires.
A powered device is an electronic device that is adapted to derive power and to receive data from such a powered network via a cable. A powered device may include a diode bridge to rectify the power supply, one or more transformer windings to isolate internal circuitry, a circuit to protect against transient over-voltage conditions, and other associated circuit components.
In general, the powered devices (PDs) may be exposed to transient high voltage conditions, which may include electrostatic discharge events, transient charges in the cable, and the like. A transient over-voltage condition refers to a high voltage level on the cable, which may be greater than a voltage rating of at least some of the associated circuit components. PoE as standardized in IEEE 802.3af dictates that PDs should be capable of withstanding such high voltage transient conditions, without sustaining high voltage-related damage.
Conventionally, PDs often include an over-voltage protection device that is separate from the integrated circuitry within the PD. In some instances, the over-voltage protection device is a high voltage transient suppressor, such as a high voltage zener diode. A typical external over-voltage protection device is a surface mount transient voltage suppressor that is rated to become active at voltage levels between 64 Volts and 70 Volts with one milliamp of current. Such a transient voltage suppressor typically clamps the supply voltage to a voltage level that is less than the voltage rating of the associated circuit components. In one particular PoE PD, the over-voltage protection device clamps the voltage at a level that is less than 94 Volts for a 4.3 A transient signal.
Unfortunately, the electrical characteristics, ratings and tolerance of the over-voltage protection device also determine the power ratings for other circuit components within the PD. For example, since 94 Volt transients are possible at 4.3 A, any coupled circuitry, such as a power regulator circuit and such as load circuitry, should be rated for a higher power level (e.g. for higher voltage and higher current levels). Unfortunately, this high voltage rating increases costs of the circuits and, consequently, increases PD unit costs.